Method of applying sulfide coating on stainless steel and composition solution therefr



Aug. 9, 1955 J. a. EAX-ren 2,715,083

METHOD OF APPLYING SULFIDE COATING ON STAINLESS STEEL AND COMPOSITION SOLUTION THEREFOR Filed Jan. 29, 1951 2 Sheets-Sheet 1 2 Sheets-Sheet 2 Aug. 9, 1955 J. E. BAXTER METHOD OF' APPLYING SULF'IDE COATING ON STAINLESS STEEL AND COMPOSITION SOLUTION THEREFOR Filed Jan. 29, 1951 /Vz'ffz'frfz//ff Z flac/127e rfa-aired far adzzzhg a2 )ez/"jays /H V4Zae5,

(F) 0,/ (H) 0.04 (G) 0.00/ (J') o.oa/ (I) 0.0/

United States Patent O METHOD F APPLYlNG SULFIDE COATING 0N STAINLESS STEEL AND COMPOSITION SOLU- TION THEREFOR This invention concerns a composition and method for the forming of a sulfide coating on a steel surface, partic-` ularly on corrosion resistant and stainless steel surfaces.

lt is known that acid solutions of sulfites and certain other sulfur compounds can be used to attack the sur face of a ferrous metal to form a sulfide coating thereon. These coatings may `serve various functions. For example, in some cases they are used as a paint base and in some cases they are used as a protective coating for the metal when the latter is subjected to wear or to extensive plastic deformation as in drawing and the like. The coatings, because of their insolubility `in acid solutions, are superior to phosphate and oxalate coatings in those uses which require contact of the coating with oils of high acidity.

In order to form such a coating, it is necessary that the solution attack the surface of the metal and form a salt of the metal. It has been pointed out, however, that the acid should not be sufliciently strong to cause excessive pickling of the metal since such pickling action, if too energetic, will prevent the forming of a4 coating. Therefore, it has been the practice to limit the acidity of the solution and to increase the rate of attack of the solution on the metal to the point desired by a suitable increase in the solution temperature.

`The problem of securing both an adequate and a controllable attackA of the acid` on the metal surface has beeny accentuatedin the case of stainless steels, which are normally quite resistant to` acid attack. The forming of sul.- fide coatings on such normally resistant metal surfaces is usually carried` out at temperatures near the boiling point of water, and, at best, inthe range` of `170--200` F. Therefore, the process has been subject tol all the disadvantages of high temperature operation; particularly the high vapor pressure of the treating solutionand the difficulty of handling acid` solutions at such` temperatures.

The term stainless steel is used herein in its generally accepted meaning. A detailed list of steels which come under the term stainless steel can be found on pages 554 and 555 of the 1948 edition of the Metals Handbook, published by the American Society for Metals.

It is an object of this invention to provide a solution anda method for the forming of a sulfide coating on the surface of a steel object, particularly on a corrosion resistant or on a stainless steel object.

A further object is to provide a strongly acid composition for the treatment of a steel surface to forma sulfide coating thereon while avoiding the excessive pickling effect normally encountered with such compositions.

It is a still further object to form a sulfide coating on a steel surface, particularly corrosion resistant and stainless steel surfaces, at temperatures much lower than those used heretofore.

I have now found that the foregoing and related objects can be secured by subjecting the surface of the stainless steel to the action of an acidic aqueous solution comprisingas essential ingredients a compound of sulfur, the fluoride ion, and a proportion of an acid sufficient to provide a pH below about 6.5; the minimum proportion of fluoride ion and the maximum pH of the treating solutions being a function of temperature.

In the sulfide coating art it has been preferred to use sulite ion containing baths. Similarly, my preferred compositions include the sulfite ion. Also, and in accord with methods disclosed heretofore, sulfide, thiosulfate', reducible organic sulfur-containing ions, etc. may be used. All such ions, other than the sulfide ion itself, form sulfide ions at the metal surface through reduction by the met-al.`

In the drawings: Figure 1 illustrates graphically the effect of added fluoride ion in lowering the temperature required to produce a satisfactory coating on an 18-8 stainless steel in selected' periods of time. Curve A illustrates a 1 hour coating and shows the fluoride limitations to be used in coating over relatively long periods of time. Curve B illustrates a 3 minute coating and represents rapid coating. The curves show the particular effect of fluoride ion when added to a coating solution containing 5 percent nitric acid and 0.32 percent sulfite ion.

As shown in the graph by curve B, .a 5 percent nitric acid solution containing 0.32 percent sulfite ion is capable of producing a suitable coating at about 172 F. in 3 minutes. The addition of even minor proportions of the added anions effects a marked reduction in the temperature required toproduce such a 3 minute coating. There is, therefore, no critical lower limit on the amount of fluoride ion to be added. Any proportion of fluoride ion, however small, offers some advantage and all proportions up to saturation are effective.

Curve A shows that if given a relatively long period of Contact, the bath can produce sulfide coatings on stainless steelI with much smaller proportions of fluoride than was required to. coat in- 3 minutes. For example, 0.04 percent fluoride is sufficient to cause a coating to form at room temperature in one hour, whereas 0.5 percent fluoride is required for rapid coating at room temperature.

The curves were determined by a large number of individual determinations and are typical of those obtained by al Variety of acids and a variety of sulfide ion producing materials.

Figure 2 illustrates the maximum pH permissible for satisfactory coatings at various temperatures. Curve C, which represents a coating in 3 minutes, is typical of rapid coating. Curve D represents 1 hour coating, which is typical of slow coatings. As shown by the curves, the maximum pH permissible is substantially proportional to the temperature. For a rapid coating the maximum pH varies from 3-.3` at room temperature to about 5 at 170 F. For slow coatings the maximum pH varies from about 4 at room temperature to about 6.5 at 170 F.

Figure 3 illustrates the minimum percentage of fluoride ion required for coating at various pH values and illustrates quite sharply the critical maximum permissible pHL Curves E, F, and G represent 3 minute coatings at F., 110 F., and 170 F., respectively. Curve E, for example, shows `that 3 minute coatings formed at extremely low pH values require about 0.5 percent fluoride ion and that as the pH is increased, somewhat more fluoride ion is required up to a pH of 3.3, above which a 3 minute coating cannot be produced at 80 F. regardless of the percentage of fluoride. Curves F and G show that an increase in temperature leads to a requirement of less fluoride at low pH- values but again a specific critical pH is reached at about 4.1 F.) and at 5 (150 F.), respectively.

Curves H and I in Figure 3 represent l hour coatings. These curves are. similar to the others except that less fluoride is required at low pH values and the critical maximum pH values are somewhatV higher. Curve I showssa` l hour coating obtained at 110 F Insofar as my invention is directed toward a composition, I have found that advantage can be gained over the compositions of the art with a composition including, in addition to a sulfur compound adapted to provide sulfide ions, the fluoride ion in any finite proportion and a proportion of an acid sufficient to provide a pH below about 6.5. However, insofar as my invention is directed toward a method of using the composition, additional advantage can be gained by restricting the lower limit of fluoride ion and the upper permissible pH to values substantially proportional to the temperature at which the method is to be carried out.

The lower limit of pH is not critical and extremely low pH solutions can be used in the presence of the fluoride ion. For example, a 60 percent sulfuric acid can be used. However, no particular advantage is gained by operating at the extreme limits of acid concentrations since the entire advantage of the invention can be obtained with more practical acid concentrations.

As indicated previously, where full advantage of the invention is desired, that is, where the composition is to be applied at the lowest possible temperature, it is necessary to place a critical limitation on the upper limit of pH. This critical limitation is quite easily determined since it is substantially proportional to the temperature selected, the reference points of the proportion being as follows: pH of about 4 at 80 F. (room temperature), pH of about 5.2 at 110 F., pH of about 6.5 at 170 F.

Where the invention provides for the rapid coating of stainless steel, the critical limitations are similarly substantially proportional to the temperature selected, the reference points being as follows: pH of about 3.3 at 80 F. (room temperature), pH of about 4.1 at 110 F., pH of about 5 at 170 F.

As pointed out previously, the upper limit of fluoride ion is not critical and fluoride ion can be used in any proportion up to saturation and, similarly, insofar as my invention is directed toward a composition, any finite proportion of fluoride, however small, contributes an advantage in that it lowers the temperature required for satisfactory coating. As in the case of the maximum pH, however, additional advantage can be carried out in the method of using the composition by placing a restriction on the lower limit of fluoride ion, which limit is substantially proportional to the temperature selected with reference points as follows: 0.001 percent fluoride ion at 170 F., 0.01 percent fluoride ion at 110 F., 0.04 percent fluoride ion at 80 F. (room temperature).

If the added advantage of securing a rapid coating is desired, the critical lower limits of fluoride ion are somewhat different but are substantially proportional to the temperature selected with reference points as follows: 0.001 percent fluoride ion at 170 F., 0.1 percent fluoride ion at 110 F., and 0.5 percent fluoride ion at 80 F.

It will be noted that the same critical lower limit of fluoride ion was given at 170 F. for both a slow coating and a rapid coating. This is for the reason that the lower limits are so small at temperatures of about 150-170 F. that the differences between them are less than the degree of analytical accuracy. Therefore, tho figure 0.001 percent fluoride ion is used to indicate both of the small, but finite, fluoride limits.

It will be apparent that the method of the invention will be most advantageously carried out rapidly and at room temperatures. To secure this advantage, it will be apparent from the foregoing that the pH of the composition to be used must be less than about 3.3 and must contain more than about 0.5 percent fluoride ion.

Although the composition of the invention requires at least about 0.03 percent of the sulfide ion producing sulfur compound, it is preferred that several tenths of 1 percent be used-for example, about 0.3 percent. Higher proportions can be used up to saturation but no particular additional advantage is secured by using such extremely high proportions as would be represented by saturation.

l 0.5 percent fluoride ion at 4 Example A 5 percent nitric acid solution containing 0.5 percent sodium sulfite and 0.5 percent fluoride ion (from potassium fluoride) was applied to the surfaceof Type 302 stainless steel for 3 minutes at room temperature. A thin adherent coating, which was a good base for paint, was obtained.

The example was repeated using a variety of acid concentrations, temperatures, and fluoride ion concentrations to establish the graphs already described in this specification. In this manner, it has been established that the particular acid, the particular source of fluoride ion, or any of the other anions is not important providing the critical limitations set forth are adhered to. In this manner, adherent sulfide coatings were produced on a wide variety of stainless steels.

I claim:

l. An acidic aqueous solution which upon contact with the surface of a stainless steel forms a sulfide coating thereon, said solution having a pH between about 6.5 and about 3.3 and consisting of water, between about 0.001 percent and saturation of the fluoride ion and between about 0.03 percent and saturation of a sulfur compound adapted to provide sulfide ions at the surface of said stainless steel.

2. An acidic aqueous solution which upon contact with the surface of a stainless steel at room temperature rapidly forms a sulfide coatingthereon, said solution having a pH of about 3.3 and consisting of water, between about 0.5 percent and saturation of the fluoride ion and between about 0.03 percent and saturation of a sulfur compound adapted to provide sulfide ions at the surface of said stainless steel.

3. The method of forming a sulfide coating on the surface of a stainless steel which comprises contacting said surface with an acidic aqueous solution at a temperature below about 170 F., said solution consisting of Water, the fluoride ion and between about 0.03 percent and saturation of a sulfur compound adapted to provide sulfide ions at the surface of said stainless steel, the minimum proportion of said fluoride ion present being substantially in inverse proportion to the temperature with reference points as follows: 0.001 percent fluoride ion at 170 F., 0.01 percent fluoride ion at 110 F., 0.04 percent fluoride ion at F., and the maximum pH being substantially proportional to the temperature with reference points as follows: pH of 4 at 80 F., pH of 5.2 at F., pH of 6.5 at 170 F.

4. The method of rapidly forming a sulfide coating on the surface of a stainless steel which comprises contacting said surface with an acidic aqueous solution at a temperature below about F., the solution con-- sisting of water, the fluoride ion and between about 0.03 percent and saturation of a sulfur compound adapted to provide sulfide ions at the surface of said stainless steel, the minimum proportion of said fluoride ion present being substantially in inverse proportion to the temperature with reference points as follows: 0.001 percent fluoride ion at 170 F., 0.1 percent fluoride ion at 110 F., 80 F., and the maximum pH being substantially proportional to the temperature with reference points as follows: pH of 3.3 at 80 F., pH of 4.1 at 110 F., pH of 5 at 170 F.

5. An acidic aqueous solution which upon contact with the surface of a stainless steel rapidly forms a sulfide coating thereon, said solution having a pH between about 6.5 and about 3.3 and consisting of water, between about 0.001 percent and saturation of the fluoride ion and between about 0.03 percent and saturation of the sulte ion.

6. An acidic aqueous solution which upon contact with the surface of a stainless steel at room temperature rapidly forms a sulfide coating thereon, said solution having a pH of about 3.3 and consisting of water, between about 0.5 percent and saturation of the fluoride ion and between about 0.03 percent and saturation of the sulte lon.

7. The method of forming a sulfide coating on the surface of a stainless steel which comprises contacting said surface with an acidic aqueous solution at a temperature below about 170 F., said solution consisting of water, the iluoride ion and between about 0.03 percent and saturation of the sulte ion, the minimum proportion of said fluoride ion present being substantially in inverse proportion to the'temperature with reference points as follows: 0.001 percent uoride ion at 170 F., 0.01 percent uoride ion at 110 F., 0.04 percent fluoride ion at 80 F., and the maximum pH being substantially proportional to the temperature with reference points as follows: pH of 4 at 80 F., pH of 5.2 at 110 F., pH of 6.5 at 170 F.

8. The method of rapidly forming a sulde coating on the surface of a stainless steel which comprises contacting said surface with an acidic aqueous solution at a temperature below about 170 F. the solution consisting of water, the fluoride ion and between about 0.03 percent and saturation of the sulte ion, the minimum proportion of said uoride ion present being substantially in inverse proportion to the temperature with reference points as follows: 0.001 percent fluoride ion at 170 F., 0.1 percent fluoride ion at 110 F., 0.5 percent fluoride ion at 80 F., and the maximum pH being substantially proportional to the temperature with reference points as follows: pH of 3.3 at 80 F., pH of 4.1 at 110 F., pH of 5 at 170 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,082,950 Green June 8, 1937 2,477,841 Ward Aug. 2, 1949 2,487,137 Hoover Nov. 8, 1949 2,500,673 Gibson et a1. Mar. 14, 1950 2,550,660 Amundsen et al May 1, 1951 2,588,234 Henricks Mar. 4, 1952 2,639,245 Baxter May 19, 1953 

1. AN ACIDIC AQUEOUS SOLUTION WHICH UPON CONTACT WITH THE SURFACE OF A STAINLES STEEL FORMS A SULFIDE COATING THEREON, SAID SOLUTION HAVING A PH BETWEEN ABOUT 6.5 AND ABOUT 3.3 AND CONSISTING OF WATER, BETWEEN ABOUT 0.001 PERCENT AND SATURATION OF THE FLUORIDE ION AND BETWEEN ABOUT 0.03 PERCENT AND SATURATION OF A SULFUR COMPOUND ADAPTED TO PROVIDE SULFIDE IONS AT THE SURFACE OF SAID STAINLESS STEEL. 